What is the white/black within a circle symbol found at mile 592.1 AHP?

At 0509, on 26 December, you pass under the Helena Highway Bridge (mile 661.7 AH...

At 0715, on March 9, you pass Knox Landing Gage at mile 313.8 (AHP) and estimate...

At 0922, on 24 May, you are abreast the St. Catherine Bar Lt. mile 348.6 (AHP)....

At 1227, on 19 October, you pass under the Greenville Highway Bridge (mile 531.3...

At 1710 on 27 November, you are abeam of Kings Point Lt. (mile 439.8 AHP). At th...

At 1723 you increase speed to make good 9.2 mph. At 1937 you have a daymark on y...

As you approach Buckridge Light (mile 412.5 AHP), which type of daymark would yo...

As you approach mile 659 (AHP), you notice on the map a dashed line crossing the...

As you approach Old River Control Structure Light you see a flashing amber light...

The charts show a circle with two black quadrants located at mile 846.0 AHP. Wha...